The present invention relates to an arcuate-tube-type Coriolis flow meter using two parallel arcuate flow tubes and to a method for determining the shape of the same.
Conventionally, there has been known a mass flow meter (Coriolis flow meter) which employs the principle that, when a flow tube through which a fluid to be measured flows and which is supported at its opposite ends is vibrated about the support points in directions perpendicular to the direction of flow within the flow tube, a Coriolis force acting on the flow tube is proportional to the mass flow rate (hereinafter, the term xe2x80x9cflow tubexe2x80x9d refers to a flow tube to which vibrations are to be imparted).
Further, according to the known art, when two flow tubes arranged in parallel are resonantly driven at mutually opposite phases, and a fluid to be measured is caused to flow through the two flow tubes at the same rate, the natural frequency of the two flow tubes is constant at all times, regardless of the type of fluid or temperature variations; thus, the Coriolis flow meter can be operated effectively and consistently while being free from influence of external vibrations and ambient temperature.
Such a conventional Coriolis flow meter using two parallel flow tubes employs a driver composed of a coil and a magnet and disposed at central portions of the flow tubes. The driver resonantly drives the two flow tubes at mutually opposite phases. A pair of vibration sensors each composed of a coil and a magnet are disposed at longitudinally symmetrical positions with respect to the installation position of the driver so as to sense a phase difference proportional to a Coriolis force.
A fluid to be measured enters the Coriolis flow meter from an external pipe connected to the flow meter via an inlet flange and branches equally into the two flow tubes. The two flows of fluid merge at the outlet side of the two flow tubes, and the resultant merged fluid flows out to an external pipe connected to the flow meter via an outlet flange. Two-parallel-flow-tube-type Coriolis flow meters are classified as being of the curved-tube type or the straight-tube type.
A curved-tube-type Coriolis flow meter employing two parallel curved flow tubes utilizes and measures vibrations of flow tube leg portions extending laterally from a body of the flow meter. Therefore, a space for the laterally extending leg portions is required, thus raising a problem of increased size.
A straight-tube-type Coriolis flow meter employs two straight flow tubes extending in the direction of an external pipe. When the straight flow tubes supported at their opposite ends are vibrated at their central portions in a direction perpendicular to their axes, the displacement difference between the supported portions of the straight flow tubes and the central portions of the straight flow tubes is sensed in the form of a phase difference signal which is used to determine the mass flow rate. Thus, the straight-tube-type Coriolis flow meter has a simple, compact, strong configuration.
However, the straight-tube-type Coriolis flow meter requires fixed support for the flow tubes at their opposite ends and is thus affected by variations in temperature. Specifically, upon change in the temperature of a fluid to be measured, the temperature of the flow tubes immediately changes accordingly, whereas a stationary structure such as a housing for securing the flow tubes exhibits delay in temperature change. As a result, a difference in elongation arises between the flow tubes and the stationary structure, thereby inducing longitudinal stress. The stress causes a change in spring constant, which in turn leads to a change in the natural frequency of the flow tubes. In order to avoid the problem, the straight-tube-type Coriolis flow meter must employ stress absorption means such as diaphragms or bellows.
The problem of longitudinal elongation induced from change in temperature can be solved through impartment of an arcuate shape to the flow tube. U.S. Pat. No. 5,796,011 discloses an arcuate flow tube. FIG. 7 is a schematic view for explaining the operation of a conventional Coriolis flow meter having an arcuate flow tube. An arcuate flow tube can disperse stress and thus exhibits excellent vibration resistance. However, a conventional arcuate flow tube is connected to a manifold in the direction aligned with the axis of an external pipe. Therefore, as shown in the upper view of FIG. 7, in the process of manufacture, the flow tube must be bent at least three times; specifically, at a central portion represented by the letter R and at two opposite end portions represented by the letter r. Such bending work is particularly disadvantageous in manufacture of a Coriolis flow meter having two flow tubes, since the two flow tubes must be symmetrical to each other. The lower view of FIG. 7 shows two states of a vertically vibrating flow tube. As shown in the lower view, even node plates, which serve as nodes of vibration, may move vertically during vibration, resulting in a failure to carry out accurate measurement.
The above-mentioned U.S. Pat. No. 5,796,011 also discloses an arcuate flow tube as shown in FIG. 8. However, such a simple arcuate flow tube fails to establish linear connection to an inlet pipe or an outlet pipe, thereby failing to provide smooth connection.
In view of the foregoing, the present invention is to provide a simple, arcuate flow tube of a single bend while establishing straight connection to an inlet pipe and to an outlet pipe for smooth connection. In order to achieve this end, desirably, as shown in FIG. 6, the arcuate flow tube consists of a central arcuate segment and two linear segments located on opposite sides of the central arcuate segment, and an inlet pipe and an outlet pipe assume a predetermined elevation angle so as to be connected to the flow tube in an aligned condition. In this case, the shape of the flow tube must be designed not only so as to be compact, but also in consideration of thermal influence.
In view of the foregoing, an object of the present invention is to determine, for a Coriolis flow meter having two parallel, arcuate flow tubes each consisting of an arcuate segment and two linear segments, the shape of the flow tube so as to suppress, to a predetermined low value, stress arising upon abrupt change in the temperature of a fluid to be measured and so as to be most compact, thereby achieving excellent stress dispersion and vibration resistance.
The present invention provides an arcuate-tube-type Coriolis flow meter and a method for determining the shape of the arcuate-tube-type Coriolis flow meter, the flow meter including two parallel flow tubes; an inlet manifold for receiving a fluid to be measured from a fluid inlet and branching the fluid to be measured into the two flow tubes; an outlet manifold for merging fluids flowing from the corresponding two flow tubes and releasing the merged fluid from a fluid outlet; a driver for resonantly driving the two flow tubes at mutually opposite phases; and a pair of vibration sensors disposed at longitudinally symmetrical positions with respect to the installation position of the driver and adapted to sense a phase difference proportional to a Coriolis force. Each of the two flow tubes assumes an arcuate shape including a central arcuate segment and two linear segments located on opposite sides of the central arcuate segment. The inside diameter of the flow tubes and the linear distance between end points of each of the flow tubes are determined on the basis of a target pressure loss arising from passage of the fluid through the manifold and the flow tube at the maximum flow rate, a target time phase difference between sine wave outputs from the paired vibration sensors at the maximum flow rate, and a target natural frequency of the flow tubes. The length of the linear segments is selected so as to reduce thermal stress induced from an abrupt change in the temperature of the fluid to be measured, and the shape of the flow tubes is determined so as to reduce the vertical height of the flow tubes, so long as the thermal stress is substantially constant even when the length of the linear segments is varied.
The present invention also provides an arcuate-tube-type Coriolis flow meter and a method for determining the shape of the arcuate-tube-type Coriolis flow meter, wherein the inside diameter of the flow tubes and the linear distance between end points of each of the flow tubes are determined on the basis of a target pressure loss arising from passage of the fluid through the manifold and the flow tube at the maximum flow rate, a target time phase difference between sine wave outputs from the paired vibration sensors at the maximum flow rate, and a target natural frequency of the flow tubes. The shape of the flow tubes is determined such that the dimensional ratio (h/L) of the vertical height (h) of the flow tubes to the linear distance (L) between the end points of each of the flow tubes is 1/4  to 1/3.